Abstract The hematopoietic system is exquisitely sensitive to radiation injury, however, little is known of the specific radiosensitivity of hematopoietic progenitor and precursors intermediates in the complex milieu of the marrow. This proposal is focused on understanding, and thus better mitigating, the effects of radiation on the erythroid and megakaryocyte lineages that originate from a common bipotential progenitor, share several cytokine growth factors, but have distinct cellular niches and unique processes of precursor maturation. We hypothesize that their biological differences lead to differential radiation sensitivity and their shared features provide opportunities for common mitigation. In Aim 1, we will determine the differential radiosensitivity of erythroid and megakaryocyte progenitors and precursors. Several cytokines are known to protect both erythroid and megakaryocyte precursors from cell death. In Aim 2, we will define the ability of single and combination cytokine therapy to mitigate radiation injury and explore whether the emergence of these two lineages from a common progenitor causes cytokine therapy to favor the mitigation of one lineage to the detriment of the other. In Aim 3, we will investigate radiation damage to the distinct niches of erythroid and megakaryocyte precursors, and thus lay the groundwork for future novel approaches of mitigating hematopoietic precursors by protecting their cellular niches. A bioterrorist attack or major nuclear disaster would lead not only to acute external radiation exposure, but may also involve secondary internal exposure through inhalation and ingestion of radioactive particulates, as has been seen in a number of nuclear incidents. In Aim 4, we will take advantage of a murine model of internal exposure recently established at the University of Rochester to begin to delineate the effects of chronic, low dose, internal radiation on the bone marrow. A better understanding of the response of the hematopoietic system and its microenvironment to radiation exposure will provide for a rational approach to its mitigation in the face of a nuclear accident or attack and, thus, enhance hematopoietic recovery and improve survival of primary victims as well their rescuers. PUBLIC HEALTH STATEMENT: The proposed research is designed to learn which blood cells in the bone marrow are sensitive to radiation and to learn how to protect these cells.